Within these few years the application of the expansive action of steam has been gradually coming into use in marine engines, and with considerable advantage; but many circumstances tend to oppose its general adoption in this class of engines, and to prevent the realization of its advantages to their full extent. Foremost amongst these obstacles is the dread which is generally entertained of the use of high-pressure steam, on account of the supposed greater danger; and so long as the general opinion on this subject remains unchanged, steam-boat proprietors will fear to employ high-pressure steam, however much they may be convinced of its superior mechanical advantage. By cutting off the steam at a quarter of the stroke, the effect is nearly doubled, but to do this requires that the pressure of the steam should be nearly four times greater than for a non-expansive engine. Larger cylinders are likewise requisite, whilst in steam vessels it is an object to reduce the bulk and weight of the machinery as far as possible. The comparatively short length of stroke in marine engines is another obstacle, as scarcely allowing sufficient time for the admission of the steam; nor is the construction of the ordinary slide valve very well adapted to working expansively.

In the Cornish engines, which are mostly pumping engines, and single acting, the valves are of the conical kind, and are each moved separately by an exceedingly ingenious apparatus, but which is not at all adapted to the use of steam vessels. In marine engines the kind of valve which is generally employed, is, as already stated, the long slide valve; the slide is moved by an eccentric, with an alternately accelerated and retarded motion, but is never absolutely at rest. Some arrangement therefore is necessary to close the steam passage, and still keep the eduction passage open. The most common plan for effecting this is, to make the blank part of the slide, or that which covers the passages, longer than the passages themselves, and to give the slide a proportionably longer stroke; but the motion of the eccentric shuts off the steam too slowly, and the two passages cannot be opened exactly at the same time, at the reversal of the stroke, so that either the eduction passage is opened too soon, or the steam passage too late.

Cams of various shapes have been substituted for the eccentric, with the view of opening and closing the passage more rapidly, and of allowing the slide to be at rest at certain points of the stroke; but they seldom work smoothly, and are generally soon destroyed by the violent concussion they experience in bringing the slide so quickly into motion from a state of rest. Some of these cams have also the defect of not being adapted to the reverse motion of the engines.

Another plan which is sometimes adopted is to construct the slide as for non-expansive engines, and to attach a separate valve between the steam pipe and the slide case. This valve is of a construction to move with little friction, and consequently requires but little force to put it in motion. It is worked by a separate cam on the paddle shaft, which can be so arranged as to close the valve at any part of the stroke, within certain limits. Although rather more complex, this plan is decidedly superior to the others, in the precision of its action, and in the circumstances that the expansion can be varied, or the engine made to work at full pressure at pleasure.